Magnetoresistive random access memory (MRAM) devices are solid state, non-volatile memory devices. A conventional MRAM device includes a column of first electrical wires (referred to as “word lines”) and a row of second electrical wires (referred to as “bit lines”). An array of magnetic memory cells are located at the junctions of the word lines and bit lines is used to record data signals.
A typical MRAM cell comprises a hard magnetic layer, a soft magnetic layer and a non-magnetic layer sandwiched between the hard magnetic layer and the soft magnetic layer. The hard magnetic layer has its magnetization vector fixed in one direction. The orientation of the fixed magnetization vectors does not change under a magnetic field applied thereon. The soft magnetic layer has an alterable magnetization vector, under a magnetic field applied thereon, that either points to the same direction (hereinafter “parallel” alignment) or opposite direction (hereinafter “antiparallel” alignment) of the magnetization vector of the hard magnetic layer. Since the resistances of the memory cell in the “parallel alignment” status and the “antiparallel alignment” status are different, the two types of alignment status can be used to record the two logical states—the “0”s or “1”s of a data bit.
In a writing operation, an electric current passes through the word line and the bit line adjacent to the memory cell. When the electric current reaches a certain threshold, the magnetic field generated by the electric currents will switch the orientation of the magnetization vector of the soft magnetic layer. As a result, the magnetization of the hard magnetic layer and the soft magnetic layer will be changed from one type of alignment (e.g. “parallel alignment”) to the other type of alignment (e.g. “antiparallel alignment”), so that a data signal can be recorded in the memory cell.
As shown in FIG. 1A, a conventional spin-valve MRAM device 100 comprises a plurality of memory cells 110, a row of bit lines 120 passing through the memory cells 110 and a column of word lines 130 passing underneath the memory cells 110. An exemplary cell 110 shown in FIG. 1B comprises a first magnetic layer 112, a non-magnetic layer 114, a second magnetic layer 116 and an antiferromagnetic layer 118. The first magnetic layer 112 is formed of ferromagnetic material such as CoFe and/or NiFe. The non-magnetic layer 114 is formed of non-magnetic material such as Cu. The second magnetic layer 116 is formed of ferromagnetic material such as CoFe and/or NiFe, and the antiferromagnetic layer 18 is formed of antiferromagnetic material such as IrMn, FeMn and/or PtMn, etc.
The above layers are disposed in sequence as shown in FIG. 1B. The second magnetic layer 116 (hereinafter “pinned layer”) has a fixed or “pinned” magnetization vector 116a pointing rightward, for example, which does not change its direction under a magnetic field applied thereon. The antimagnetic layer 118 serves to fix the magnetization of the second magnetic layer 116.
The first magnetic layer 112 has a magnetization vector 112a that is alterable under a magnetic field applied thereon. During a writing process, a word line current 132 passing underneath the cell 110 and a bit line current 122 passing through the cell 110 generate a magnetic field on the first magnetic layer 112. When the magnetic field reach the switching threshold of the first magnetic layer 112, the orientation of the magnetization vector 112a of the first magnetic layer 112 will be changed from pointing leftward to pointing rightward. With the orientation change, the magnetization vector 112a becomes parallel to the magnetization vector 116a, which represents a low magnetic resistance state of the cell 110. When the word line current 132 and the bit line current 122 flow in a direction opposite to that of the example above, the magnetization vector 112a will be changed to become anti-parallel to the magnetization vector 116a. This represents a high magnetic resistance state of the cell 110. The “low” and “high” states correspond to binary data bits “0” and “1” by which a data signal may be stored in the memory cell. The signal may be read or detected when a “read” current passes through the bit line.
One problem encountered in this type of memory cell is that the second magnetic layer 116 generates a static field and applies on the first magnetic layer 12, which tends to disrupt the stability of the magnetization of the first magnetic layer 112. As the dimensions of the memory cells are decreased to achieve a higher data storage capacity, this static field effect will only become more significant.
For example, when the dimensions of the memory cells shrink, external thermal fluctuations will cause the magnetization vector of the first magnetic layer 112 to rotate, which may cause data recording errors. Another problem encountered in this type of memory cell is that with the size of the memory cells scaled down, there generates a large edge domain in the first magnetic layer 112, which also contributes to the thermal unstability of the memory device.
Various solutions have been developed to address the above problems. In “Spin Valve Sensors With Synthetic Free and Pinned Layers” to Anabela et al (J. Appl. Phys. Vol 87, Num. 9, 5744, 2000), a spin-valve memory element comprising synthetic layers is disclosed. The synthetic layers are used to eliminate the interlayer magnetostatic coupling, decrease pinned layer demagnetizing field and reduce the effective thickness of the free layer, as shown schematically in FIG. 2.
The memory element 200 comprises a synthetic ferrimagnetic free layer 210 having an alterable effective magnetization vector 210a, a non-magnetic layer 220 disposed above the free layer 210, a pinned layer 230 and an antiferromagnetic layer 240. The pinned layer 230 further includes three sub-layers 232, 234 and 236, and the free layer 210 further includes three sub-layers 212, 214 and 216.
In this structure, the ferromagnetic sub-layer 232 and 236 are antiferromagnetically coupled to each other through a non-magnetic layer 234, and the ferromagnetic sub-layer 212 and 216 are antiferromagnetically coupled to each other through a non-magnetic layer 214. The magnetization of the ferromagnetic sub-layer 236 is configured anti-parallel to that of the first ferromagnetic sub-layer 232, therefore the static field generated by the pinned layer 230 and applied onto the free layer 210 can be reduced. In addition, as the magnetization of the ferromagnetic sub-layer 212 is anti-parallely aligned with that of the ferromagnetic sub-layer 216, the effective thickness of free layer 30 can be reduced.
While the thermal stability may be increased in this type of memory device, another problem arises as since the synthetic ferromagnetic layers required a larger writing current to perform the writing operation, the power dissipation of this type of device increases.
U.S. Pat. No. 6,358,757 to Anthony et al discloses a method for forming a magnetic memory with a set of structures that prevent disruptions to the magnetization in the free layer of a magnetic memory cell. The structure includes a high permeability magnetic film that serves as a keeper for the sense layer magnetization. In this structure, however, the large edge domain effect which contributes to the thermal instability of the device, remain unsolved.
It is therefore desirable to provide a magnetic memory device having a higher resistance against thermal fluctuations so that to improve the thermal stability and reduce the edge domain effect. While maintaining a high thermal stability, there is in the meantime a need to have a magnetic memory device having a higher sensitivity in response to the writing current, so that to reduce the overall power dissipation.